maybe title isnt entirely clear but i wonder if a faraday cage style mesh could be used inside a plastic case in order to trap emissions? and protect from external noise?
say i develop a system, put it in a plastic box, but its in a noisy environment, and is itself noisy.. so wouldnt pass any of the EMC law/tests were it to be tested.. would it be possible to get an aluminium mesh on the inside of the casework and would that then work to shield internal and external emissions?

It will depend very much on the mesh size. A very fine mesh may de the job. But a mesh will always leak. It is much better to use solid materials. This information are for cables but it should give you some idead regarding this matter. Most cable use braided shields. Braids typically provide only 60% to 98% coverage and are less effective as shields than solid conductors. Braided shields usually provide just slightly reduced electric field shielding. Except at UHF frequencies. But greatly reduced magnetic field shielding. The reason is that braid distorts the uniformity of the longitudinal shield current. A braid is typically from 5 to 30 dB less effective than a solid shield for protecting against magnetic fields. At higher frequencies, the effectiveness of the braid decreases even more as a result of the holes in the braid.

nice find!
i had found a fine aluminium mesh for auto repair which i argued could be easily folded around the plastic case, perhaps soldered to the ground plane of the device... or not i would imagine not, still.. i shall have to find out! get my 3d printer up and running first

We spent a lot of time developing Faraday shields for our metal detectors to minimize capacitive effects on the antennas and also add a degree of shielding to them. In one case, we used a fine copper mesh bonded in place so it couldn't move but the detector could scan thru it. In other instances, a painted surface of graphite collodion, grounded at one end proved very effective. Surface resistivity was about 1 kohm at one cm distance as I recall. The biggest problem that we had to solve was static electricity discharge which of course, would trip the metal detector falsely; ie (no metal). The Faraday shield provided a partial solution for this problem by providing a bleed-off path to ground.

But the circuit design/routing,etc.. should be evaluated first.. Sometimes ferrite beads,bypass caps,ground planes,trace/track lengths,etc.. are all that's needed to pass EMI/EMC testing.
A few simple board changes "could" prevent you from reoccurring costs associated with additional shielding,etc...

Finding an engineer that is well versed in EMI/EMC techniques and have him look it all over. That 1 time cost might be well worth it.